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Food Waste Management and Utilization
INTRODUCTION TO FOOD WASTE MANAGEMENT
Food waste refers to the discarding or loss of food throughout the various stages of the food
supply chain, from production and processing to distribution and consumption. It can occur for
various reasons, including overproduction, spoilage, inefficient harvesting, and consumer
behavior. Food waste is a significant global issue with environmental, economic, and social
consequences. It contributes to greenhouse gas emissions, wasted resources, and exacerbates
food insecurity. Efforts to reduce food waste involve interventions at each stage of the supply
chain and raising awareness about responsible consumption.
Food waste is food that is not eaten. Food waste can be categorized into four types:
• By-product food waste
• Expired products
• Leftovers
• Bakery and packaged food waste
The FAO defines food waste as. “…food appropriate for human consumption being
discarded, whether it’s kept beyond its expiry date or left to spoil”.
Food loss is the decrease in the quantity or quality of food resulting from decisions and actions
by food suppliers in the chain, excluding retailers, food service providers and consumers
Food that is lost during post harvesting, storage and transportation level before packaging or at
retail levels.
1. Food Loss: Food loss occurs primarily during production, post-harvest handling, and storage.
It refers to the decrease in the quantity or quality of food resulting from decisions and
actions by food suppliers in the chain, excluding retailers, food service providers, and
consumers. Examples include crops left in the field due to spoilage, damage during
harvesting, or inadequate storage facilities.
2. Food Waste: Food waste encompasses the discarding or loss of food at the retail and
consumer levels. It refers to food that is edible but is not consumed for various reasons. This
can include food that is left uneaten on plates in restaurants, food that expires on shelves in
supermarkets, or leftovers that are thrown away at home.
Food waste management refers to the practices and strategies that deal with food that gets lost or
discarded throughout the food supply chain. It's all about preventing food waste, but also finding
ways to utilize unavoidable scraps and leftovers responsibly. Here's a breakdown of the key aspects:
Goals:
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• Reduce food waste: This is the primary aim, focusing on preventing food spoilage,
inefficiencies in production and distribution, and overconsumption.
• Recover and utilize food waste: When food waste can't be prevented, the goal is to find
alternative uses. This could involve donating food nearing expiry to food banks, using food
scraps for animal feed, or composting them for fertilizer.
• Responsible disposal: If food waste can't be recovered, disposing of it responsibly is
important. This might involve converting it into bioenergy or disposing of it in landfills in a
way that minimizes environmental impact.
FOOD BY PRODUCTS AND ITS TYPES:
Food byproducts are secondary products that come from primary agro-food production
processes. They can be a cheap source of functional ingredients like peptides, carotenoids, and
phenolic compounds.
TYPES
Food industry waste refers to the food that goes to waste during production, distribution, and
when people don't finish what they buy. About one-third of all food made globally, which is roughly
1.3 billion tons, gets lost or thrown away each year. This happens a lot in both poor and rich
countries, often because of problems like bad farming practices, tough weather, and not having
good ways to move food around. Food waste causes big problems for the environment, money, and
people's access to food.
The food industry generates significant waste at various stages of production, distribution, and
consumption. This waste can be categorized into several types:
1. Raw Material Waste: Happens when the materials used to make food don't meet quality
standards or there's too much of them.
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2. Processing Waste: Occurs when making food into finished products, like scraps or leftover
bits.
3. Packaging Waste: Comes from the containers and wrapping used to package food, especially
if it's too much or can't be recycled.
4. Distribution Waste: This is waste that happens during moving food around or storing it, like
when food spoils or breaks during transport.
5. Retail Waste: Happens when stores can't sell all their food, whether it's because they
bought too much, people didn't want it, or it went bad.
6. Consumer Waste: Occurs when people throw away food at home because they bought too
much, didn't store it right, or let it go bad.
7. Post-Consumer Waste: Refers to food thrown away after it's bought, like when restaurants
or catering services have leftovers.
Food waste can be split into two main types:
1. Avoidable Waste: This is the part of food waste that could be prevented or reduced by using
better practices or management. It includes things like making too much food, not storing it
properly, or people not eating what they buy. Examples include bread, milk, fruits, and
vegetables that were thrown away even though they were still good to eat. To tackle
avoidable waste, we need to manage inventory better, improve how we move food around,
teach people to be more responsible with what they buy and eat, and use technology to
predict what we need more accurately.
2. Unavoidable Waste: This is the waste that happens even if we're doing everything right to
try to reduce it. It includes parts of food that we can't eat, like bones or peels, and losses
that happen during processing that we can't avoid practically or economically. Unavoidable
waste can be composted or recycled to reduce its impact on the environment. Examples
include meat bones, eggshells, fruit skins, and tea bags. To reduce unavoidable waste, we
can find other uses for these by-products, like using them for animal feed or turning them
into energy through special processes like anaerobic digestion. Making this distinction helps
us figure out the best ways to reduce food waste overall.
The environmental impacts of food waste are significant and diverse, contributing to
various environmental issues such as:
➢ Greenhouse Gas Emissions: Greenhouse gas emissions are a major environmental consequence
of food waste. When food is thrown away and ends up in landfills, it decomposes anaerobically
(without oxygen), producing methane gas. Methane is a potent greenhouse gas, with a global
warming potential many times higher than that of carbon dioxide. The decomposition of organic
matter in landfills is one of the largest sources of human-related methane emissions globally.
Methane traps heat in the atmosphere, contributing to global warming and climate change. By
reducing food waste, we can help decrease methane emissions and mitigate the impacts of
climate change.
➢ Resource Depletion: Producing food requires various natural resources, including water, land,
and energy. When food is wasted, these resources are also wasted. For example, producing one
kilogram of beef requires about 15,000 liters of water. When that beef is wasted, all that water is
effectively wasted as well.
➢ Energy Consumption: The production, transportation, and storage of food all require energy.
When food is wasted, the energy used to produce, transport, and store it is also wasted.
Reducing food waste can help conserve energy and reduce the associated environmental
impacts.
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➢ Water Pollution: The use of pesticides and fertilizers in food production can lead to water
pollution. When food is wasted, the chemicals used in its production are also wasted,
contributing to water pollution and harming aquatic ecosystems.
➢ Landfill Space: Food waste takes up valuable space in landfills, which are already under pressure
from increasing amounts of waste. Landfill space is finite, and expanding or creating new landfills
can have detrimental effects on local ecosystems and communities.
➢ Loss of Biodiversity: Agriculture and food production often lead to habitat destruction and
biodiversity loss through deforestation, land conversion, and the use of pesticides and fertilizers.
Wasting food exacerbates these environmental impacts by increasing the demand for
agricultural land and intensifying production practices.
➢ Water Scarcity: Food production is a major consumer of freshwater resources. Wasting food
means wasting the water used to grow, process, and transport it, contributing to water scarcity,
particularly in regions already facing water stress or scarcity
Economic implications of food waste
1. Production Costs: Farmers invest resources in cultivating, harvesting, and transporting crops.
When a portion of these crops goes to waste, it leads to a direct loss of investment for the farmers,
impacting their profitability.
2. Processing and Packaging Costs: Food processing and packaging industries incur expenses in
preparing and preserving food items. If these products are wasted, the costs associated with their
processing and packaging become essentially wasted as well.
3. Distribution Costs: The transportation and distribution of food involve considerable expenses.
When food is wasted, it means that the resources spent on getting it from the farm to the consumer
are lost, affecting the efficiency of the entire distribution system.
4. Retail Costs: Retailers face losses due to unsold perishable goods, affecting their profit margins.
Additionally, they may need to implement markdowns or discounts on products nearing expiration,
further impacting revenues.
5. Consumer Expenses: From the consumer perspective, food waste contributes to higher grocery
bills. When food is thrown away at home, consumers essentially pay for items that provide no
nutritional value.
6. Environmental Costs: While not direct economic costs, the environmental impact of food waste
has economic implications in the long run. The resources used in production, transportation, and
disposal contribute to environmental degradation, and addressing these issues may require
economic investments.
Social implications of food waste
1. Hunger and Food Insecurity: One of the most glaring consequences is the exacerbation of global
hunger. While a significant portion of the world's population struggles with food insecurity, the
wastage of edible food highlights a stark contrast and ethical dilemma.
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2. Economic Disparities: Food waste contributes to economic disparities as affluent societies often
discard significant amounts of food, while many individuals and communities cannot afford an
adequate diet. This economic inequality is evident both globally and within individual countries.
3. Environmental Impact: The social implications are intertwined with environmental concerns.
Food waste generates substantial greenhouse gas emissions due to the decomposition of organic
matter in landfills. This, in turn, exacerbates climate change, which disproportionately affects
vulnerable communities.
4. Resource Utilization: The production of food involves the use of various resources such as land,
water, and energy. When edible food is wasted, it implies a squandering of these resources. In a
world with a growing population and increasing resource constraints, this inefficiency is socially
irresponsible.
5. Ethical Considerations: Food waste raises ethical questions about the distribution of resources
and the values embedded in a society. It prompts discussions about responsibility, fairness, and the
moral obligation to address hunger and poverty.
6. Health Implications: The social repercussions of food waste also extend to health. Nutrient-rich
foods are often discarded, contributing to dietary imbalances. Additionally, the overproduction and
wastage of food may encourage unhealthy consumption patterns.
7. Community Dynamics: Food waste impacts community dynamics by influencing the relationships
among producers, consumers, and distributors. A more conscientious approach to food
consumption fosters a sense of responsibility and interconnectedness within communities.
8. Educational Opportunities: Addressing food waste provides opportunities for education and
awareness. Initiatives to reduce waste can lead to increased knowledge about sustainable practices,
nutrition, and the interconnectedness of global food systems.
Principles of food waste management
1. Prevention: This is the most preferred and cost-effective strategy, focusing on reducing food
surplus at every stage of the supply chain. It entails measures to avoid generating excess
food that would eventually become waste.
2. Minimization: Minimization involves efforts to decrease the overall volume of waste
generated. This can include optimizing production processes, improving inventory
management, and implementing portion control measures to reduce excess food.
3. Reuse: Reuse entails finding alternative uses for food items or products nearing the end of
their intended lifespan. This could involve repurposing ingredients or products, donating
surplus food to communities in need, or utilizing food waste for composting or animal feed.
4. Recycling: Recycling involves transforming food waste into new products or materials. This
can include composting organic waste to create nutrient-rich soil amendments or utilizing
anaerobic digestion to generate renewable energy from organic matter.
5. Disposal: Disposal, particularly in landfills, is the least favorable option and should be
considered only after exhausting all other alternatives. Landfill disposal contributes to
environmental pollution and greenhouse gas emissions, making it imperative to prioritize
prevention and diversion efforts.
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Methods of food waste management
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7. Biogas Generation: Organic waste, such as food scraps, animal by-products, or organic
residues from food packaging industries, undergo a transformation process at
biodegradation plants. Here, through the action of bacteria, fungi, or other microorganisms,
this waste is converted into biogas. These microorganisms feed on the organic matter,
breaking it down either aerobically (in the presence of oxygen) or anaerobically (without
oxygen). The resulting biogas is utilized as a fuel source, while the remaining residue serves
as valuable manure.
8. Vermicomposting: Vermicomposting harnesses the power of worms to decompose organic
materials into nutrient-rich compost. Worms ingest and digest the organic matter, releasing
by-products that enrich the soil with essential nutrients. This process fosters the growth of
beneficial bacteria and fungi, enhancing soil health and promoting plant growth.
Vermicomposting is recognized for its efficiency and effectiveness compared to traditional
composting methods.
Benefits of Food Waste Management:
Effective food waste management offers numerous environmental, economic, and social
advantages:
1. Environmental Impact:
o Reduced Methane Emissions: By diverting food waste from landfills and utilizing
methods such as composting or anaerobic digestion, methane emissions, a potent
greenhouse gas, can be significantly reduced.
o Conservation of Resources: Proper management prevents the wasteful use of
resources like water, energy, and land that are typically expended in food
production and transportation.
2. Economic Savings:
o Lower Disposal Costs: Municipalities and businesses can save on disposal expenses
by diverting food waste from landfills.
o Potential Revenue Streams: Methods such as anaerobic digestion can generate
biogas, offering opportunities for energy production and potential revenue.
3. Social Benefits:
o Food Redistribution: Surplus food that would otherwise be discarded can be
redistributed to those in need, addressing food insecurity and poverty.
o Community Engagement: Initiatives aimed at reducing food waste foster community
involvement, raise awareness, and encourage sustainable behaviors.
4. Resource Recovery:
o Composting: Food waste composting produces nutrient-rich soil amendments,
reducing reliance on chemical fertilizers and promoting soil health.
o Anaerobic Digestion: Organic waste converted into biogas and nutrient-rich
digestate through anaerobic digestion provides renewable energy and fertilizer.
5. Climate Change Mitigation:
o Carbon Footprint Reduction: Managing food waste helps lower the overall carbon
footprint associated with food production, transportation, and disposal.
6. Regulatory Compliance:
o Meeting Waste Reduction Targets: Effective food waste management aids in
achieving waste reduction goals set by regulatory authorities and governmental
bodies.
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7. Reduces Greenhouse Gas Emissions: When food waste decomposes in landfills, it produces
methane, a potent greenhouse gas. By managing food waste through composting, anaerobic
digestion, or other methods, methane emissions can be significantly reduced.
8. Conserves Resources: Food production requires valuable resources such as water, land, and
energy. By minimizing food waste, these resources are conserved, contributing to
sustainability efforts.
9. Saves Money: For households, businesses, and communities, reducing food waste can lead
to significant cost savings by lowering food purchasing and disposal costs.
10. Supports Food Security: Redirecting surplus food to those in need through food banks,
shelters, or other charitable organizations helps combat food insecurity in communities.
11. Creates Renewable Energy: Anaerobic digestion of food waste can generate biogas, a
renewable energy source that can be used for electricity generation, heating, or
transportation fuels.
ENVIRONMENTAL BEST- PRACTICE TECHNOLOGIES FOR WASTE MINIMIZATION.
1. Hydrothermal Carbonization (HTC): This process subjects food waste to high temperatures
and pressure in the presence of water, converting it into hydrochar, a solid fuel resembling
coal. Hydrochar can be used for energy production in various applications such as power
generation or heating.
2. Dendro Liquid Energy (DLE): DLE technology utilizes a thermochemical process to convert
food waste into a liquid fuel, which can be used similarly to traditional fossil fuels. This liquid
fuel can power vehicles, generators, or heating systems, offering a renewable alternative to
conventional energy sources.
3. Ultra-fast Hydrolysis: This innovative method rapidly breaks down complex organic
molecules in food waste into simpler components like sugars and amino acids. These
components can then be utilized in various industrial processes, including biofuel production
or as feedstock for biochemical manufacturing.
4. Anaerobic Digestion: Anaerobic digestion employs microorganisms to break down food
waste in an oxygen-free environment, producing biogas (methane and carbon dioxide) and
nutrient-rich digestate. Biogas can be utilized as a renewable energy source for electricity or
heat generation, while digestate can be used as organic fertilizer.
5. Composting: Composting is a natural biological process where microorganisms decompose
organic matter, including food waste, into humus-like material known as compost. Compost
enriches soil fertility, improves soil structure, and promotes plant growth, making it valuable
for agricultural and horticultural applications.
6. Pretreatment: Pretreatment technologies prepare food waste for further processing
methods such as hydrothermal carbonization or anaerobic digestion. This may involve
grinding, shredding, or dehydrating the waste to enhance its suitability for efficient
conversion into useful products.
7. Precision Agriculture: Precision agriculture employs various technologies, including sensors,
GPS, drones, and data analytics, to optimize farming practices. By precisely managing
resources like water, fertilizers, and pesticides, precision agriculture minimizes waste,
increases crop yield, and reduces environmental impact.
8. Smart Packaging: Smart packaging incorporates sensors, indicators, and antimicrobial
coatings to extend the shelf life of food products, reducing spoilage and waste. Time-
temperature indicators, freshness sensors, and oxygen absorbers are examples of smart
packaging technologies that help maintain food quality throughout the supply chain.
9. Cold Chain Management: Cold chain management ensures the integrity of perishable goods
by maintaining specific temperature conditions during storage and transportation. This
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prevents premature spoilage and extends the shelf life of food products, reducing waste
along the supply chain.
10. Food Processing Technologies: Advanced processing techniques such as freeze-drying,
vacuum packaging, and high-pressure processing preserve the nutritional value and quality
of food products while extending their shelf life. By minimizing spoilage and microbial
contamination, these technologies help reduce food waste throughout the manufacturing
and distribution process.
11. Supply Chain Visibility: Supply chain visibility technologies track the movement of food
products from production to consumption, providing real-time information on inventory
levels, location, and conditions. This enables efficient inventory management, reduces losses
due to expiration or damage, and optimizes distribution to minimize waste.
12. Food Redistribution Platforms: Digital platforms connect surplus food from producers,
retailers, and restaurants with food banks, shelters, or redistribution organizations. By
diverting edible food away from landfills and redistributing it to those in need, these
platforms reduce food waste and contribute to food security efforts.
13. Smart Waste Monitoring Systems: Sensor-based waste monitoring systems track waste
generation, composition, and fill levels in real-time, allowing for proactive waste
management strategies. By optimizing collection routes, scheduling, and resource allocation,
these systems minimize waste generation and improve operational efficiency.
14. Educational Technologies: Educational tools and platforms raise awareness about food
waste among consumers, businesses, and communities. Interactive apps, online courses,
and educational campaigns provide information on food conservation, proper storage,
portion control, and composting, encouraging responsible consumption habits and waste
reduction practices.
15. Automated Sorting Systems: Advanced sorting technologies such as optical sorting,
magnetic separation, and robotics automate the separation of organic waste from other
materials in waste management facilities. By improving recycling rates and diverting organic
waste for composting or anaerobic digestion, these systems minimize landfill disposal and
promote resource recovery.
16. Food Waste Tracking Apps: Mobile applications enable businesses and consumers to track
and manage their food consumption, inventory, and waste generation. By providing insights
into purchasing patterns, expiration dates, and portion sizes, these apps help minimize over-
purchasing, reduce food waste, and save money.
Characteristics of industrial waste
• Physical characteristic (Order, temperature ,color ,total solids ,turbidity)
• biological characteristics
• chemical characteristics (Measurement of organic compound, organic matter and inorganic matter)
• toxicity
PHYSICAL CHARACTERISTICS
Total Solids Total solids refer to the amount of solid matter present in a given volume of industrial
waste. This measurement includes both suspended solids (particles that are insoluble and remain in
suspension) and dissolved solids (solutes that are dissolved in the liquid phase).
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1. Order: Industrial waste can be classified into three main categories based on their physical state:
a. Solid waste: This includes materials such as paper, plastics, metals, glass, and construction debris.
Solid waste can be further subdivided into hazardous and non-hazardous waste, depending on its
potential to harm human health or the environment.
b. Liquid waste: This category includes wastewater, effluents, and sludge generated by industrial
processes. Liquid waste can contain a wide range of contaminants, such as heavy metals, organic
compounds, and nutrients.
c. Gaseous waste: This refers to air emissions from industrial processes, which can include particulate
matter, volatile organic compounds (VOCs), and greenhouse gases.
2. Temperature: The temperature of industrial waste can vary widely, depending on the specific process
and materials involved. High-temperature waste may require special handling and treatment methods
to prevent fires, explosions, or other hazards.
3. Color: The color of industrial waste can provide clues about its composition and potential hazards. For
example:
a. Clear or colorless waste may indicate the presence of water, solvents, or other low-concentration
contaminants.
b. Dark or opaque waste may contain high concentrations of suspended solids, organic matter, or
other contaminants.
c. Bright or fluorescent waste may indicate the presence of chemicals or dyes.
4. Total Solids: This refers to the total amount of solid material in a waste sample, expressed as a
percentage of the total volume or weight. High total solids content may indicate a higher
concentration of pollutants or contaminants, which could affect the choice of treatment and disposal
methods.
5. Turbidity: This measures the cloudiness or haziness of a liquid waste sample, which can be an
indicator of the presence of suspended solids, organic matter, or other contaminants. High turbidity
may require additional treatment steps, such as sedimentation, filtration, or chemical coagulation, to
remove suspended particles and improve the clarity of the wastewater.
• A measure of the light-transmitting properties of water due to the presence of colloidal &
residual suspended matter Measurement based on comparison of the intensity of light
scattered by a sample to the light scattered by a reference suspension under the same
conditions.
CHEMICAL CHARACTERISTICS of industrial waste refer to the presence and concentration of various
organic and inorganic compounds, which can have significant impacts on the environment and
human health. Some key chemical characteristics include:
1. Organic compounds: These are compounds that contain carbon-hydrogen bonds and can be
further classified into volatile organic compounds (VOCs), semi-volatile organic compounds
(SVOCs), and persistent organic pollutants (POPs). Examples of organic compounds in industrial
waste include solvents, pesticides, and hydrocarbons.
2. Organic matter: This refers to the total amount of carbon-containing compounds in a waste
sample, including both biodegradable and non-biodegradable materials. High levels of organic
matter in wastewater can lead to oxygen depletion in receiving waters, causing harm to aquatic
life.
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3. Inorganic matter: This includes compounds that do not contain carbon-hydrogen bonds, such as
heavy metals, nutrients, and salts. Inorganic compounds in industrial waste can include toxic
elements like lead, mercury, and cadmium, as well as nutrients like nitrogen and phosphorus
that can contribute to eutrophication in water bodies.
To assess the chemical characteristics of industrial waste, various analytical methods are employed,
such as:
1. Gas chromatography (GC): This technique is used to separate and quantify volatile organic
compounds in waste samples.
2. High-performance liquid chromatography (HPLC): This method is used to separate and
quantify semi-volatile and non-volatile organic compounds in waste samples.
3. Atomic absorption spectroscopy (AAS): This technique is used to measure the concentration
of heavy metals and other inorganic elements in waste samples.
4. Total organic carbon (TOC) analysis: This method is used to determine the total amount of
organic carbon in a waste sample, which can be an indicator of the presence of organic
matter.
Biological characteristics of industrial waste refer to the presence and concentration of
microorganisms, pathogens, and other biological agents that can pose risks to human health and the
environment. Some key biological characteristics include:
1. Pathogens: These are disease-causing microorganisms, such as bacteria, viruses, and
parasites, that can be present in wastewater and other industrial waste streams. Examples
of pathogens in industrial waste include Salmonella, E. coli, and Legionella.
2. Indicator organisms: These are non-pathogenic microorganisms that are used as surrogates
to estimate the potential presence of pathogens in waste samples. Examples of indicator
organisms include fecal coliforms, fecal streptococci, and Enterococcus.
3. Nutrients: These are essential elements for microbial growth and reproduction, such as
nitrogen, phosphorus, and potassium. High levels of nutrients in wastewater can contribute
to eutrophication in receiving waters, leading to algal blooms and oxygen depletion.
4. Biodegradable organic matter: This refers to the fraction of organic matter in waste that can
be broken down by microorganisms, releasing nutrients and other compounds into the
environment. High levels of biodegradable organic matter can lead to oxygen depletion in
receiving waters and contribute to the growth of pathogenic microorganisms.
5. Antibiotic-resistant bacteria: These are microorganisms that have developed resistance to
one or more antibiotics, making them more difficult to treat and control. The presence of
antibiotic-resistant bacteria in industrial waste can pose significant risks to public health and
the environment.
6. COD (Chemical Oxygen Demand) is a measure of the amount of oxygen required to oxidize
organic and inorganic compounds in a waste sample. It serves as an indicator of the potential for
wastewater to consume oxygen in receiving waters, which can lead to oxygen depletion and harm to
aquatic life.
COD is determined by subjecting a waste sample to a strong oxidizing agent, such as potassium
dichromate, and measuring the amount of oxygen consumed during the reaction. The higher the COD
value, the greater the potential for oxygen depletion and the more severe the environmental impact.
TOXICITY
Toxicity refers to the ability of a substance to cause harm or damage to living organisms, including
humans, animals, and plants. In the context of industrial waste, toxicity is an important characteristic
to consider, as it can have significant impacts on human health and the environment.
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Wastewater treatment
Wastewater treatment is the process of removing contaminants from wastewater, including
household sewage and industrial effluent, to produce treated water that is safe to be released back
into the environment or reused for various purposes. This process typically involves physical,
chemical, and biological methods to remove pollutants such as solids, organic matter, nutrients,
pathogens, and toxic substances. The goal of wastewater treatment is to protect public health,
safeguard the environment, and conserve water resources.
Steps of sewage treatment process:
I. Preliminary treatment of wastewater:
▪ The main objective of preliminary treatment is to remove gross solids (such as plastics, cloths,
cans, dead body of animals etc), grits and fats from waste water.
▪ Some of the treatment technique applied for preliminary treatment purpose are;
i. Screening:
▪ Screening is used to remove gross solid waste like plastics, cloths, dead animals from waste
water.
▪ For this purpose waste water is passed through a metal screen which consists of vertical or
inclined steel bars usually set 5 cm apart.
▪ The removes gross solids are disposed by burning or composting.
ii. Grit removal:
▪ Grits are small, non-biodegradable particles which are heavier than suspended organic
matters.
▪ Grits are removed by carefully regulating the flow velocity of sewage in grit removal tank
iii. Skimming:
▪ Skimming is the process of removal of fatty and oily material from sewage.
▪ In this method, sewage is placed in skimming tank and it is aerated from bottom so that fats
and oils are collected at top of the liquid which are then removed by skimming.
II. Primary treatment of wastewater:
i. Sedimentation:
▪ Sedimentation tank is used for removal of suspended solids and some organic matters.
▪ There are different types of sedimentation tank.
▪ Common example is rectangular horizontal flow tank. In this tank sewage flow very slowly (1-2
feet/min) such that solids present is waste water settle at bottom.
▪ Settled solids are periodically removed by sludge scrapper.
▪ This technique removes about 90% of suspended solids and about 40% of organic matters from
sewage.
ii. Mechanical flocculation:
▪ In this method sewage is paced in a flocculation tank, then sewage is rotated at an speed of
0.43m/sec with the help of rotating paddles
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▪ While sewage rotates in circular motion, small size dissolved solids attached to each other to form
large size solids and settles at the bottom which is then removed out.
iii. Chemical flocculation:
▪ In this method, sewage is placed in coagulation tank and then some precipitating agents such as
alum is added.
▪ Alum forms precipitate of Al(OH)3, suspended solids attached to the precipitate such that size of
precipitate increase gradually to settle down at bottom.
iv. Neutralization:
▪ If sewage is highly acidic or basic, it is neutralized by adding base or acid to facilitate growth of
microorganisms during secondary treatment process.
III. Secondary treatment of waste water:
i. Trickling filter:
▪ Trickling filter consists of filtering bed, spraying arm and water collecting chamber.
▪ Filtering bed consists of well graded gravel, broken stone of size (40-150mm diameter).
▪ Effluent or sewage from primary treatment tank is sprayed uniformly over the filter bed. During
filtration a gelatinous layer of bacteria, algae, protozoa and some fungi is produced on the surface of
filter bed. This layer is called Zoogleal layer.
▪ As the water trickles through the filter bed, organic matter present in it are oxidized by microorganism
of zoogleal layer.
▪ Although trickling filter is classified as aeration process of sewage treatment, it is facultative system. It is
because aerobic bacteria lies on the top of the filter bed whereas anaerobic bacteria lies in middle or
bottom of filter bed.
▪ Trickling filter can reduce BOD of sewage by about 65-85% depending on the rate of filtration.
ii. Oxidation ditch:
▪ Oxidation ditch consists of circular canal with inlet and outlet.
▪ In this method, sewage from primary treatment plant is placed in oxidation ditch and then it is agitated
with the help of mechanical rotator and then left for a period of about 12-24 hours.
▪ During the period of oxidation, microorganism present in sewage oxidize the organic matter.
▪ Finally the sewage is removed from oxidation ditch through outlet for tertiary treatment.
▪ Oxidation pond or lagoon:
▪ Oxidation pond is also known as lagoon or reduced pond or stabilization pond.
▪ It is an aerobic method of sewage treatment technique.
▪ In this treatment method, sewage from primary treatment plant is placed in an oxidation pond and left
there for 10-40 days.
▪ During this period in oxidation pond, microorganisms oxidize the organic matter present in sewage.
Oxygen released by algae during photosynthesis is utilized by microorganism for oxidation of organic
compounds. During oxidation CO2 and H2O are released which are utilized by algae for photosynthesis.
Therefore there is mutually beneficial relationship between algae and bacteria.
▪ Some oxygen is also derived from atmosphere for oxidation because oxidation pond is open system.
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▪ The oxidation pond remains aerobic during day time and first hours of night. During this period
oxidation of organic compound (aerobic decomposition) takes place. During rest hours of night
condition become anaerobic and anaerobic decomposition of organic compound takes place.
▪ Advantage of oxidation pond;
▪ It is very simple and easy technique
▪ Treated sewage can be utilized for irrigation
▪ Limitation of oxidation tank:
▪ Holding time is very long (10-40 days)
▪ It require large area
▪ It creates bad odor. Furthermore it may become breading place for mosquitoes and other
vectors
▪ It is influenced by seasonal temperature. It is effective only in warm climate but not in cold and
rainy season.
iii. Activated sludge system:
▪ Activated sludge system, consists of aeration tank, settling tank and sludge return system.
▪ At first sewage from primary treatment plant is mixed with sludge drawn from previous batch, which is
known as activated sludge or return sludge.
▪ The activated sludge contains large number of microorganisms and serves as inoculum of
microorganisms.
▪ After mixing of activated sludge, sewage is placed in aeration tank. In aeration tank. Sewage is
continuously aerated for 6-8 hours. During this period microorganisms oxidizes the organic compounds
to form CO2, H20 and NO3 etc.
▪ After oxidation, sewage is passed to settling tank and left undisturbed for 2-3 hours. Sludge settle to the
bottom. This sludge is called activated sludge which is fully oxidized and is very offensive. This activated
sludge can be used as inoculum for next batch of sewage.
▪ Most of the sludge is removed and some is returned to aeration tank for next round of treatment.
▪ By sludge digestion process, BOD of sewage is reduced by 5-15%.
iv. Septic tank:
▪ Septic tank is used for disposal of content of toilet where sewage system is not available for disposal.
▪ Septic tank is prepared under the ground.
▪ Sewage along with toilet content is placed into septic tank where heavier solid wastes settle down to
from sludge whereas lighter solids including fats form layer on top of sewage called scum.
▪ In septic tank organic compounds in sewage is anaerobically digested by anaerobic microorganisms
such as Methanogenic bacteria.
▪ After anaerobic decomposition, the sludge become stable and inoffensive whereas liquids in sewage
percolates into soil from septic tank.
IV. Tertiary or final treatment of waste water:
i. Removal of suspended solids:
▪ Suspended solids are removed by two methods:
▪ Microstraining:
▪ In this method, sewage is placed in rotating drum filter of pore size 25-35 µm and then
drum is rotated,
16 | P a g e N O T E S B Y H A Y
▪ During rotation, clear water comes out of drum and suspended solids remains inside
drum.
▪ Chemical coagulation and filtration:
▪ Chemical coagulation and filtration:
▪ In this method, precipitating agents such as alum is added in sewage. Fine
suspended solids adsorbs to the surface of Al(OC)3 precipitate, finally precipitate
with adsorbed solids are separated by filtration.
ii. Removal of dissolved solids like salts:
▪ Various technique are used for this purpose such as adsorption by activated carbon, reverse osmosis
▪ Adsorption by activated carbon:
▪ Dissolved solids can be removed by filtering the water through filter containing activated
carbon particle.
▪ Reverse osmosis:
▪ Reverse osmosis removes dissolved solids like NaCl and microbial cells
iii. Removal of nitrate and phosphate
▪ If sewage after treatment is to be discharge into river, nitrate and phosphate should be removed from
sewage before disposal. It is because nitrate and phosphate causes eutrophication.
▪ These plant nutrients are removed by biological process. At first sewage is placed in a tank containing
nitrifying bacteria. These bacteria converts ammonium salt and nitrite into nitrate
▪ Then the sewage is placed into second tank containing denitrifying bacteria. These bacteria converts
nitrate into Nitrogen gas that leaves the sewage.
▪ Phosphate is also removed by bacteria by microbial assimilation process.
iv. Killing of microorganisms
▪ Finally microorganisms in sewage are killed by disinfection like chlorination.
